The present invention relates to control systems, and more particularly, to a method for determining a specific condition which causes an interlock to activate (or sometimes referred to as causing an interlock to trip).
All of the products on market today for implementing interlocks provide interlock logic displays that only show the current interlock conditions but provide no indication of which one of the many possible interlock conditions actually was responsible for causing an interlock trip.
Discrete devices in process control systems (e.g., pumps, motors, valves, etc.) are usually prevented from operating unsafely or incorrectly by means of interlocks. Process conditions, usually called interlock conditions, are conditions such as "furnace temperature greater than a SAFE value" or "boiler pressure too high" or "two of four pumps not operating for over 10 minutes". These are combined with logic, (most often, but not restricted to, Boolean or relay ladder logic language), usually called the interlock logic, in order to "force" the device to a SAFE state (if not already in that state) based on the process situation, and keep it in that state until the process conditions causing the interlock are returned to normal. The change of the device's operating state (e.g., stopping of a pump which was running, or opening of a pressure relief valve, etc.) due to interlock is often called interlock trip and not being allowed to operate is usually referred to as the device is interlocked.
Following an interlock trip it is important to identify the cause of the trip. Frequently the condition responsible for causing the trip may no longer exist and must be captured so the operator, possibly at a later time, can identify the cause and take the necessary corrective action.
Thus there is a need to provide a method whereby the cause of the interlock trip can be identified and captured so that it can be presented to the operator in a convenient manner and time. There is provided by the present invention a method in which the specific process condition that caused the interlock trip is identified by tracing back through the interlock logic and saved, such that even if the actual condition subsequently clears (or becomes OK) or if additional conditions (which may be different from the conditions at the time of the initial interlock trip) are now present the operator can unambiguously tell the original cause of the interlock trip from the operational display of the discrete device without resorting to system logs or journaling mechanisms.